This invention relates to improved polyester monofilaments, and is more particularly concerned with monofils of polyethylene terephthalate which have suitable straight tenacity and transverse properties for reinforcing elastomeric products.
The multifilament yarns conventionally used as reinforcement in elastomeric articles have been composed of low denier filaments of polyethylene terephthalate or nylon. The filaments deniers have ranged from 3 to 12, and large numbers of filaments have been combined by plying, twisting and cabling to obtain cords of sufficient size for use as reinforcing elements. For example, tire cord deniers are in the range of 1500 to 2000, or higher, and V-belts may be reinforced with cords having total deniers of 8000 to 10,000, or more.
The disadvantages of plying, twisting and cabling low denier filaments would be avoided by the use of large denier monofilaments, if monoflaments could be produced to have equivalent properties. Mitsuishi et al. U.S. Pat. No. 3,651,198 issued Mar. 21, 1972, discloses a process of drawing and then heat-treating low denier filament of 0.8 to 0.9 intrinsic viscosity polyethylene terephthalate to produce yarns having breaking strengths and elongations of 11.8 gpd./20% to 13.5 gpd/16%. The patent states, "When used as tire cord, they show excellent fatigue resistence and high strength-conversion ratio." Example 17 illustrates that an undrawn monofilament of 1200 denier can be drawn 6.2 X in two stages, at 65.degree. and 45.degree.C., and then heat-treated in three stages at 65.degree., 120.degree. and 180.degree.C. with 10-15% stretching in each stage, to provide a monofil of less than 200 denier having a breaking strength of 13.8 gpd. with a break elongation of 16%. The patent does not teach that a combination of high strength and fatigue resistance can be obtained in monofilaments which have deniers as large as the cords used in tires or V-belts.
It has been found that processes suitable for low denier filaments do not produce equivalent properties in large denier monofilaments. Cuculo U.S. Pat. No. 2,880,057, issued Mar. 31, 1959, discloses a process for improving the properties of monoflaments in which an undrawn filament of 20 mil. diameter is formed by extruding high molecular weight polyethylene terephthalate through an air gap into a quench tank, containing water at room temperature, and the undrawn filament is passed through a second air gap and into a heater where the filament is heated under substantially zero tension to a temperature between 160.degree. and 210.degree.C. to crystallize the polymer. The heat-treated filament is cold drawn in a liquid bath at up to 6 X draw ratio and is then heat-set at 190.degree.C. The patent states that filaments are obtained which have good tensile strength and good fatigue resitance in toothbrush bristles. Surprisingly, it has been found that the above crystallizing step before drawing precludes drawing of high denier monofilaments intended for use in tires or V-belts.
Hansen U.S. Pat. No. 3,650,884, issued Mar. 21, 1972, discloses a process for producing nylon monofilaments having good tenacity and good loop tenacity. Comparison tests as reinforcement plies in tires showed the performance of such monofilaments of polyhexamethylene adipamide to be equivalent to that of commercial 66-nylon cord. The nylon monoflaments are extruded downwardly through an orifice, through a conventional air gap (i.e., usually less than 2 inches), and quenched in water in conventional manner. The quenched monofilament is drawn in two coupled stages between feed and draw rolls, using a draw ratio of about 4 X in the first stage and about 1.5 X in the second stage. Between each pair of feed and draw rolls, the monofilament passes through a radiant oven heated to between 500.degree. and 750.degree.C. The drawn monofilament is then passed through a steaming chamber where it is exposed for 0.2 to 1.0 second to steam at a temperature above the melting point of the polyamide in the steam atmosphere. This steaming produces a modified outer layer on the monofilament and is show to provide a marked improvement in loop tenacity without decrease in straight tenacity. Although steaming is effective for nylon, the steaming process disclosed does not improve the loop tenacity of polyester monofilaments.